Project Summary Deep venous thrombosis (DVT) and secondary pulmonary embolism affect 0.1-0.2% of the population and cause 60,000-100,000 deaths annually, an incidence and mortality similar to that of myocardial infarction. In 1856 the pathologist Rudolph Virchow implicated changes in venous blood flow in DVT pathogenesis, but a molecular and genetic basis for how hemodynamic changes drive DVT pathogenesis has not been identified, and present therapy is restricted to prophylactic measures to augment venous blood flow and systemic anticoagulation. We have recently demonstrated that the endothelial GATA2-FOXC2-PROX1 transcriptional pathway is activated by oscillatory or reversing flow and required to stimulate the formation of venous and lymphatic valves. Our preliminary studies demonstrate that endothelial cells around venous valves that experience similar oscillatory flow express the GATA2-FOXC2-PROX1 transcriptional program in association with a strong anti-coagulant phenotype marked by low vWF, high EPCR, high TM, and high TFPI expression. Loss of this transcriptional program conferred by altered venous flow or genetic deletion in peri-valvular ECs results in clot formation around the venous valve. We hypothesize that endothelial GATA2-FOXC2- PROX1 expression stimulated by oscillatory flow maintains an anticoagulant endothelial phenotype required to prevent DVT formation. This proposal will test this hypothesis using a combination of genetic approaches to specifically delete the GATA2/FOXC2/PROX1 pathway in mouse peri-valvular endothelial cells, surgical approaches to reduce venous flow in the mouse, and histologic and physiologic studies of human venous valves to test whether this mechanism is conserved in humans and lost during DVT pathogenesis. These studies are expected to establish a genetic and molecular mechanism for DVT pathogenesis that will serve as the foundation for novel mechanical and molecular therapies.